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ORIGINAL ARTICLE
Year : 2022  |  Volume : 12  |  Issue : 3  |  Page : 309-315

Root canal morphology and incidence of isthmus in the mesial root canal of the mandibular first molar using cone-beam computed tomography of two different scanning modes


Department of Endodontic, Faculty of Dentistry, Suez Canal University, Ismailia, Egypt

Date of Submission07-Mar-2022
Date of Decision30-Apr-2022
Date of Acceptance01-May-2022
Date of Web Publication1-Sep-2022

Correspondence Address:
Dr. Hayam Yousef Hassan
Suez Canal University, El Sheikh Zayed, El Salam District, Ismailia
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/sej.sej_54_22

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  Abstract 

Introduction: The mesial root of the mandibular molar is one of the most complex internal anatomies of human dentition. This study aimed to detect root canal morphology and the incidence of isthmus in the mesial root canal of the mandibular first molar using cone-beam computed tomography (CBCT) of different voxel size scanning modes compared to stereomicroscopic evaluation after teeth clearance.
Materials and Methods: Sixty extracted mandibular first molars were collected, fixed in foam blocks, and scanned using CBCT with 150 and 400 μm voxel sizes. All the teeth were cleared and fixed in clear epoxy blocks for serial cross-sectioning from the apex, then examined using a stereomicroscope. Comparisons between cross-sectioned slices were achieved using Friedman's test (P ≤ 0.05).
Results: There was a statistically significant difference between the cleared teeth, voxel sizes of 400 and 150 μm in the detection of the root canal and isthmus. Only voxel size 150 μm was able to detect the isthmus at 1 mm and 2.5 mm from the apex. Vertucci Types IV and II were the most common canal configurations reported, with a high incidence of isthmus at 4 mm and 5.5 mm from the apex.
Conclusion: The 150 μm voxel size mode provides more accurate detection of the isthmus portion and root canal morphology. Vertucci Types IV and II were the most common canal configurations, and an isthmus with a high incidence was found at 4 mm and 5.5 mm from the apex of the mesial root of the mandibular first molar.

Keywords: Cone-beam computed tomography, isthmus portion, mandibular molars, root canal morphology, teeth clearance technique


How to cite this article:
Zaher MN, Rabie MI, Hassan HY. Root canal morphology and incidence of isthmus in the mesial root canal of the mandibular first molar using cone-beam computed tomography of two different scanning modes. Saudi Endod J 2022;12:309-15

How to cite this URL:
Zaher MN, Rabie MI, Hassan HY. Root canal morphology and incidence of isthmus in the mesial root canal of the mandibular first molar using cone-beam computed tomography of two different scanning modes. Saudi Endod J [serial online] 2022 [cited 2022 Oct 5];12:309-15. Available from: https://www.saudiendodj.com/text.asp?2022/12/3/309/354836


  Introduction Top


Accurate knowledge of root canal morphology and its anatomical variations is essential for successful surgical and nonsurgical root canal treatment.[1] The success of root canal treatment is primarily determined by knowledge of the root canal system morphology, as well as successful negotiation, cleaning, shaping, and obturation. Knowledge of root canal anatomy is the foundation upon which subsequent procedures are built.[2] Several studies have focused on root anatomy using various methods such as radiography, sectioned teeth, computed tomography (CT), scanning electron microscope, and clearance of teeth with ink injection.[3],[4],[5] Several methods, such as the clearance technique, transverse cross-sectioning, and scanning electron microscope, are used to detect the internal anatomy of teeth. The teeth clearance technique is considered the gold standard method.[6]

Teeth clearance technique should include three steps such asdecalcification, dehydration, and clarification. Nitric acid (HNO3) in a high concentration is an effective inorganic decalcifying agent. Five percent Indian and Chinese ink are the most widely used materials for injection to differentiate the root canal anatomy. Methyl salicylate is used as an immersion oil to clarify the teeth for microscopic examination, owing to its refractive index.[7]

Isthmus is an important anatomical feature defined as a narrow ribbon-shaped communication between two root canals that may contain necrotic tissues, pulp remnants, microorganisms, and their by-products.[8] Neglecting the isthmus may lead to inadequate root canal obturation and failure of root canal treatment.[9] Hsu and Kim[10] classified isthmus of the root canals into five categories: Type I comprised two or three canals without visible communication; Type II two canals with a visible connection; Type III presence of a canal between the two main canals (incomplete C-shaped canals); Type IV extension of the main canal into the isthmus area; and Type V complete connection or corridor of the two canals throughout the section.

The two-dimensional radiographs could not regularly reveal the definite number of root canals.[11],[12] However, the most accurate, noninvasive technique is cone-beam CT (CBCT) that provides different resolution and allows three-dimensional imagining of anatomical structures.[11]

Most studies have focused mainly on using CBCT for determining tooth morphology and numbers of root canals,[12] but few studies have assessed the isthmus portion.[13]

A factor that could possibly influence the accuracy of the CBCT images is the voxel size scanning mode, which can be considered tiny cubes arranged next to each other. Each voxel is a value that represents the X-ray density of the corresponding structure. However, voxel size depends on scanning time that exposes the patient to a higher radiation dosage. Therefore, the voxel size has an influence on the linear accuracy of CBCT.[14]

This study aimed to detect root canal morphology and the incidence of isthmus in the mesial root canal of mandibular first molar using CBCT in two different scanning modes 150 and 400 μm voxel size, compared to stereomicroscopic evaluation after teeth clearance.


  Materials and Methods Top


This in vitro blinded study was approved by the Research Ethics Committee of the Faculty of Dentistry, Suez Canal University (No. 206/2019).

Sixty unidentified mandibular first molars extracted for periodontal reasons were randomly selected from a group of molars, including permanent molars with intact apices, no extensive restorations, or previous root canal treatment, and molars with calcified root canals, root caries, or internal or external root resorption were excluded.

Simple randomization was performed by generating numbers from 1 to 60 using a randomizer (https://www.randomizer.org/).[15] All teeth, including canal configuration and isthmus detection, were recorded by the main operator. The operator did not have to be blinded to the teeth, as all procedures were performed on all teeth according to the manufacturer's instructions. However, all observers evaluated root canal morphology from the microphotographs or CBCT images in a blinded manner.

Cone-beam computed tomography scanning

All teeth were arranged in two blocks of polystyrene foam,[16] and a metal ball was inserted into the block on the mesiobuccal side to identify the mesial and buccal sides of the CBCT image. All teeth were scanned twice with a CBCT machine (Planmeca ProMax 3D Mid machine; Helsinki, Finland), first in standard mode (400 μm voxel size) and second in high-resolution mode (150 μm voxel size). The data were compared with those of cleared teeth. The standard and high-resolution modes differed only in voxel size, whereas the other parameters (mA, Kvp, focal spot size, exposure time, and field of view) were standardized.

In both modes, one sagittal and five axial sections were obtained from each mesial root. The number and types of root canals were identified from sagittal sections of the root canals according to the classification of Vertucci.[6] The type of the isthmus was determined according to the classification of Hsu and Kim.[10]

Teeth clearance technique

Access cavities were done using round diamond (Mani Inc., Japan) and endo-Z burs (Dentsply, Maillefer, USA). The teeth were demineralized by soaking in 5% HNO3 (EL; Gomhoria Co., Egypt) at room temperature, and the HNO3 solution was changed daily for 2 days.[17] Decalcified teeth were washed for 1 h by running tap water to remove acid remnants.[18] The samples were dehydrated by ascending concentrations of ethyl alcohol (80%, 90%, and 98%) for 1 day. The teeth were placed in a disposable plastic irrigation syringe with a 27-gauge needle (Ameco Co., Egypt) containing Indian ink (Speedball 2, Onza Tinta, China; Negro Super.) until the ink penetrated into the root canal system under negative pressure. Dehydrated teeth were immersed in methyl salicylate 99% (EL; Gomhoria Co., Egypt), which rendered them transparent.[19],[20]

For each tooth, diamond disks (Kerr Dental, Orange, CA, USA) were used for hemi-sectioning of the teeth at the furcation area to remove their distal roots. The mesial root was fixed in a clear epoxy resin block. Each block was sectioned perpendicular to the long axis of the root obtaining five cross-sections 1.0, 2.5, 4.0, 5.5, and 7.0 mm distant from the apex using an automatic saw machine with a continuous coolant (Isomet 4000 linear precision saw, Illinois, USA).[21]

Both transparent teeth and cross-sectioned slices were examined with the naked eye and a stereomicroscope (Nikon MA 100, Japan).[22] Sagittal sections were examined at ×10 while axial (cross) sections at ×40. Similar magnifications were used for the CBCT data. The following observations were made: (1) number of root canals (defined as root canal orifices in the pulp chamber), (2) root canal configuration in each root by using Vertucci's classification,[6] and (3) presence of isthmus communications between the two mesial root using Hsu and Kim classification.[10]

Three endodontists with at least 3 years of experience evaluated the root canal morphology and isthmus portion classification from CBCT images. The obtained results were compared with those of cleared sections.[16]

Statistical analysis

Qualitative data were showed as frequencies and percentages. Comparisons between CBCT in two different scanning modes 150 and 400 μm voxel size, and teeth clearance technique was achieved using Friedman Test. The significance level was fixed at P ≤ 0.05. Statistical analysis was done using IBM SPSS Statistics for Windows, version 23.0 (Armonk, NY, USA: IBM Corp).


  Results Top


Root canal classification

There was a statistically significant difference found between the root canal types measured at voxel size of 150 um and 400 μm and clearance (P < 0.001, effect size = 0.150). A voxel size of 400 μm is associated with the highest prevalence of unclear images. A voxel size of 150 μm and clearance showed the same prevalence in Type II and IV canal configurations, whereas a voxel size of 400 μm showed a lower prevalence in Type II and IV. There was no difference in the detection of Type VI for voxel sizes of 150 and 400 μm or clearances [Table 1] and [Figure 1], [Figure 2].
Table 1: Descriptive statistics and results of Friedman's test for comparison between root canal type according to Vertucci's classification and configuration as measured by voxel sizes 150 μ, 400 μ, and clearance

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Figure 1: Sagittal section photographs of mesial root of mandibular first molar showing Vertucci type IV: (a) cleared root under stereomicroscope, (b) CBCT scanning using 150 μm mode, and (c) CBCT scanning using 400 μm mode

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Figure 2: Sagittal section photographs of mesial root of mandibular first molar showing Vertucci type VI: (a) cleared root under stereomicroscope, (b) CBCT scanning using 150 μm mode, and (b) CBCT scanning using 400 μm mode.

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Isthmus classification at different levels

Mode voxel size 150 μm

There was a statistically significant difference found between the isthmus type at the different levels (P < 0.001, effect size = 0.353). The highest prevalence of Hsu and Kim classification Type I isthmus was found at the 7 mm level followed by the 2.5, then 1 mm levels, and Type II was found only at the 4 mm level.

The highest prevalence of Type IV was found at level 5.5 mm followed by 4 mm and none was shown at levels 1 and 7 mm, while Type V was recorded at the 4 mm level followed by the 5.5 and 2.5 mm levels. The highest prevalence of unclear isthmus was recorded at the 1 mm level, followed by all other levels with the same prevalence [Table 2] and [Figure 3].
Table 2: Descriptive statistics and results of Friedman's test for comparison between isthmus classifications at different levels as measured by voxel size 150 μ

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Figure 3: Photographs showing cross-sections for the same mesial root at different levels from the apex, scanned by CBCT using 400 μm and 150 μm modes, then cleared and examined under stereomicroscope: (a) Isthmus Type IV at 1 mm. (b) Isthmus Type IV at 2.5 mm. (c) Isthmus Type V at 4 mm. (d) Isthmus Type IV at 5.5 mm. (e) Isthmus Type IV at 7 mm. CBCT: Cone-beam computed tomography

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Mode voxel size 400 μm

There was a statistically significant difference found between the isthmus type at the different levels (P < 0.001, effect size = 0.334). The highest prevalence of Type I isthmus was recorded at the 7 mm level and the lowest at the 1 mm level.

Type II was detected only at the 4 mm level. The highest prevalence of Type IV was noted at 5.5 mm and Type V at 4 mm, while levels 1 and 2.5 mm of Type IV and level 1 and 7 mm of Type V did not show isthmuses.

The highest prevalence of unclear isthmus was found at the 1 mm level followed by 2.5, then 4 mm. The lowest (and same) prevalence was found at 5.5 and 7 mm levels [Table 3] and [Figure 3].
Table 3: Descriptive statistics and results of Friedman's test for comparison between isthmus classifications at different levels as measured by voxel size 400 μ

Click here to view


Teeth clearance technique

There was a statistically significant difference found between the isthmus type at the different levels (P < 0.001, effect size = 0.320). The highest prevalence of Type I isthmus was recorded at the 1 mm level followed by the 7, 2.5, then 5.5 mm levels, and the lowest was found at the 4 mm level.

Type II was found only at the 4 mm level. The highest prevalence of Type IV was found at the 5.5 mm level and none at 1 mm level, while the highest prevalence of Type V was recorded at the 4 mm level and none at 1 and 7 mm levels. An unclear isthmus was only found in one sample at the 1 mm level, while all other levels did not show an unclear isthmus [Table 4] and [Figure 3].
Table 4: Descriptive statistics and results of Friedman's test for comparison between isthmus classifications at different levels as measured by teeth clearance technique

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The incidence of the isthmus at all levels was significantly different between the presence of the isthmus, as measured by 150 μm and 400 μm, and clearance (P = 0.001, effect size = 0.302). Clearance showed the highest prevalence of the isthmus (85%), followed by a voxel size of 150 μm. A voxel size of 400 μm (76.6%) showed the lowest prevalence of isthmus (48.3%) [Table 5].
Table 5: Descriptive statistics and results of Friedman's test for percentage of isthmus as measured by voxel sizes 150 μ, 400 μ, and clearance

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  Discussion Top


The present study was performed using the mesial root of the mandibular first molar because it is characterized by flattened mesiodistal and widened buccolingual surfaces that allows a high incidence of isthmus between the root canals that present different types of root canal morphology.[23],[24]

For many decades, the tooth clearance technique is the best-accepted method to study variations of root canal morphology.[8] It allows for three-dimensional visualization of the tooth's internal structures. This method renders the tooth transparent through demineralization and injecting fluid materials such as ink.[25] The main limitation of this technique is that it produces irreversible changes in the tooth structure and may create artifacts.[26]

This study aimed to find the best clinical method that shows the actual anatomy of root canals; therefore, two different resolutions of CBCT, 150 and 400 μm voxel size, were selected as interventions to evaluate the morphology of the root canals, such as the isthmus, which is not visible on two-dimensional radiographs.[27]

The voxel size was an important parameter to support the reliability of the study of root and canal anatomy. The lower the voxel size, the more reliable the outcome and the higher the exposure dose.[28] CBCT devices with larger voxel sizes decrease the resolution. Therefore, voxel size 150 μm was the most effective in detection of the root canal number and isthmus in a specific root or tooth.[18]

Although, notable radiation reduction that may be derived from voxel size 400 μ must be considered as an option in our evaluation. The increase in resolution must be evaluated against the increase in radiation to the patient.[29]

Some CBCT machines have lower voxel size than 150 μ, like the endo mode in some devices with voxel size 70 μ. However, it is not available in all CBCT machines, more expensive, and requires a higher radiation dose.[30]

All teeth were embedded in epoxy blocks, and serial cross-sectioning was performed from the apex as a reference point. The cuts were made at 1.5 mm intervals 1.0/2.5/4.0/5.5/7.0 mm distant from the apex as the sections were 1 mm in thickness and the cutting disc was 0.5 mm in thickness.[21]

This study found that both the high-resolution mode and tooth clearance technique detected the root canal classifications better than the standard mode, as reported by Ji et al.[16] Furthermore, 51.7% of the root canals were Vertucci Type IV, which is in agreement with both the teeth clearance technique study carried out by Gulabivala et al.[31] and micro-CT study of Marceliano-Alves et al.[32]

In the present study, 1 mm and 2.5 mm from the apex showed the lowest incidence of isthmus, which agrees with the findings of Lima et al.[21] and Ji et al.,[16] as only clearance and a voxel size of 150 μm could explain the isthmus, whereas a voxel size of 400 μm showed the highest prevalence of unclear images at these areas. In addition, CBCT can be considered for root canal evaluations; however, some morphological aspects and voxel size can influence the ability of CBCT to detect certain features.[33]

Our results showed the highest prevalence of isthmus at the levels of 4 mm and 5.5 mm from the apex and that strongly agreed with Teixeira et al.[34] serial cross-sectioning study on 50 mandibular first molar. Furthermore, isthmus Type IV and Type V were found more than Type II and Type III and that agreed with the findings of Von Arx[35] who examined the root end resection of 56 mandibular first molars with an endoscope.

The limitations of this in vitro study are that it did not represent a real clinical situation where no artifacts were produced, such as the presence of metal coronal restoration close to the observed tooth or internal resorption, in addition to the small number of evaluated teeth.

Further studies with larger sample sizes, use of different types of CBCT units, and comparison with other methods of evaluation such as micro-CT are needed.


  Conclusion Top


The 150 μm voxel size mode was accurate in detecting the isthmus portion of the root canal as well as the root canal morphology. Vertucci Types IV and II were the most common canal configurations, and isthmus with a high incidence was found at 4 mm and 5.5 mm from the apex in the mesial root of mandibular first molar.

Acknowledgments

This study was supported by Suez Canal University and the Department of Endodontics. The authors declare no potential conflicts of interest with respect to the authorship and/or publication of this article.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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